Use of CO2 /steam to enhance floods in horizontal wellbores

ABSTRACT

A method to enhance steam flooding where at least two horizontal wellbores are utilized. Carbon dioxide is injected into a lower perforated horizontal wellbore. Once sufficient carbon dioxide has been injected into the formation, steam is injected through the lower horizontal wellbore. The steam displaces the carbon dioxide into the formation where it contacts and mixes with hydrocarbonaceous fluids. Steam causes the carbon dioxide to expand, thereby providing for a better sweep of the formation. Steam injection is ceased and liquid carbon dioxide injection again is commenced. Afterwards, steam is injected again into the formation. This sequence is continued until it becomes uneconomical to produce hydrocarbonaceous fluids from an upper horizontal wellbore. Hydrocarbon displacement efficiencies are enhanced when hydrocarbons are produced into the upper horizontal wellbore due to viscosity and density differences.

FIELD OF THE INVENTION

This invention is directed to a method for carbon dixoide/steamstimulation of hydrocarbonaceous fluids via at least two horizontalwellbores. More particularly, it is directed to the use of liquid carbondioxide and a subsequent steam flood which causes the expansion ofcarbon dioxide so as to obtain a substantially better sweep of aformation containing said horizontal wellbores.

BACKGROUND OF THE INVENTION

With advances in drilling technology, it is currently possible to drillhorizontal wellbores deep into hydrocarbon producing reservoirs.Utilization of horizontal wellbores allows extended contact with aproducing formation, thereby facilitating drainage and production of thereservoir.

Although horizontal wellbores allow more contact with the producingformation, some difficulties are encountered when horizontal wellboresare utilized which are not commonly experienced when vertical wells areused. Methods used in producing hydrocarbons from a formation orreservoir via vertical wells often prove to be inefficient whenattempting to remove hydrocarbons from a reservoir where horizontalwellbores are being used. This inefficiency results in utilization ofincreased amounts of fluids used during enhanced oil recovery operation.This results in a dimunition in the amount of hydrocarbons removed fromthe formation or reservoir.

This inefficiency is demonstrated when a carbon dixoide flood isutilized with a vertical wellbore where the formation contains zones ofvarying permeability. Often the carbon dioxide overrides a zone of lowerpermeability leaving hydrocarbonaceous fluids behind.

U.S. Pat. No. 4,736,792, issued to Brown et al. on Apr. 12, 1988,discloses a method for treating a well completed in a subterraneanformation containing petroleum where a preconditioning process wasemployed. The preconditioning process was used to improve thereceptivity of the formation to steam. The method involved injecting aheated non-condensible and oil soluble gas, in the gaseous phase, intothe formation so as to avoid permanently fracturing the formation andalso avoid the immediate formation of an oil bank.

Stephens in U.S. Pat. No. 4,607,699, issued Aug. 26, 1986, discusses ahuff-puff cyclic steam stimulation method. Here a formation is fracturedby liquid carbon dioxide injection. While carbon dioxide is still inplace within the formation, steam is injected into the formation. Aftera suitable soaking period, the well is opened to production.

Therefore, what is needed is a method to improve the sweep efficiency ofliquid carbon dioxide in a formation where only horizontal wellbores areutilized and the formation is not fractured.

SUMMARY OF THE INVENTION

This invention is directed to a method for the removal ofhydrocarbonaceous fluids from a formation which is penetrated by atleast two horizontal wellbores. In the practice of this invention,liquid carbon dioxide is injected into a lower horizontal wellbore whereit enters the formation and contacts hydrocarbonaceous fluids therein.While the liquid carbon dioxide is in the formation, steam is injectedinto the lower horizontal wellbore so as to cause the liquid carbondioxide to be heated and expand. Heating also causes the carbon dioxideto go into its gaseous state and make additional contact withhydrocarbonaceous fluids in the formation. Any carbon dioxide thatremains undissolved in the formation is driven deeper into the formationby the steam where it makes additional contact with thehydrocarbonaceous fluid-containing formation. Pressure exerted by thesteam and the carbon dioxide causes a hydrocarbonaceous/carbon dioxidefluid mixture to form which proceeds upwardly through the formation intoan upper horizontal wellbore. The mixture of hydrocarbonaceous fluids,gaseous carbon dioxide, steam, and water exits the formation through theupper horizontal wellbore where it is produced to the surface. Uponreaching the surface, the hydrocarbonaceous fluids are separated fromthe carbon dioxide, steam and water.

It is therefore an object of this invention to increase the verticalrelative permeability of a formation in which at least two horizontalwellbores have been placed for the removal of hydrocarbonaceous fluids.

It is another object of this invention to use liquid carbon dioxide anda steam flood in a formation containing at least two horizontalwellbores so as to maximize drainage of the formation.

It is yet another object of this invention to provide for liquid carbondioxide stimulation of the formation in combination with steam so thatthe formation can be stimulated with any length of a horizontalwellbore.

It is a still further object of this invention to provide for a liquidcarbon dioxide/steam flood method which can enhance oil recovery from aformation via at least two horizontal wellbores.

BRIEF DESCRIPTION OF THE DRAWING

The drawing is a schematic representation showing displacement offormation oil by expanded carbon dioxide where two horizontal wellboresare utilized.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the practice of this invention as is shown in the drawing, wellbore12 penetrates a hydrocarbonaceous fluid-bearing formation 10.Hydrocarbonaceous fluids in said formation can have a gravity of fromabout 10 to about 60 API degrees. At its lower end wellbore 10 isdeviated in a manner so as to form a lower horizontal wellbore 28 whichcontains perforations 14 on its topside. At a desired distance fromhorizontal wellbore 28 is placed an upper horizontal wellbore 26 whichhas perforations 14 on its bottomside. Horizontal wellbore 26 is fluidlyconnected to wellbore 12. The angle of deviation from vertical wellbore12 for both horizontal wellbore 26 and horizontal wellbore 28 is about10° to about 90°. Tubing 16 is centered in the vertical portion ofwellbore 12 by packer 18 so as to cause fluid communication by tubing 16with only lower horizontal wellbore 28. Tubing 16 being centered inwellbore 12 and held in place by packer 18 forms annulus 24 in wellbore12 which annulus fluidly communicates with upper horizontal wellbore 26only.

In order to remove hydrocarbonaceous fluids from formation 10, liquidcarbon dioxide is injected into tubing 16 where it flows into formation10 via perforations 14 contained in lower horizontal wellbore 28. Oncein formation 10, the temperature of formation 10 causes some of theliquid carbon dioxide to form a gas which penetrates the formation andmixes with hydrocarbonaceous fluids contained therein. A portion of theliquid carbon dioxide dissolves in the oil lowering the oil's viscosityand causing the formation contacted to be more receptive to steampenetration. Any undissolved liquid carbon dioxide that remains in theformation is driven deeper into formation 10 by a subsequent steamflood. Liquid carbon dioxide is injected into the formation at a rateand volume which will not fracture the formation. Once sufficient liquidcarbon dioxide has been injected into formation 10, injection of liquidcarbon dioxide into formation 10 is ceased. A method for injectingliquid carbon dioxide into formation 10 is disclosed in U.S. Pat. No.4,607,699, issued to Stephens on Aug. 26, 1986. This patent is herebyincorporated by reference. The teachings of this patent can be utilizedso long as the fracturing pressure of formation 10 is not exceeded byliquid carbon dioxide injection.

After all the liquid carbon dioxide has been injected, steam injectionis commenced. A method for injecting steam into the formation isdiscussed in U.S. Pat. No. 4,607,699, as mentioned above. Steam isinjected via tubing 16 into lower horizontal wellbore 28 by perforations14 where it enters formation 10. Steam injection is continued until asufficient amount of steam has been directed into the formation. Whenthe steam contacts the liquid carbon dioxide 20, it converts the liquidcarbon dioxide into its gaseous state whereupon it mixes withhydrocarbonaceous fluids in formation 10 and is pushed outwards towardupper wellbore 26. When the mixture comes into contact with wellbore 26,it enters perforations 14 and exits wellbore 26 via annulus 24 and isremoved from the formation by wellbore 12 to the surface. After removingthe carbon dioxide/hydrocarbonaceous fluid mixture from the formation,it is separated from the carbon dioxide, steam and water.

Displacement efficiencies in directing hydrocarbonaceous fluids to theupper horizontal wellbore 26 are enhanced by injecting liquid carbondioxide again into the formation. Once sufficient liquid carbon dioxidehas been injected into the formation, injection of carbon dioxide isceased and steam injection once again commenced. This sequence isrepeated until the desired amount of hydrocarbonaceous fluids has beenremoved from the formation.

As will be understood by those skilled in the art, although an upper andlower wellbore are shown in the drawing communicating fluidly with thevertical section of wellbore 12, individual horizontal wellbores can beutilized. A separate lower horizontal wellbore can be used as aninjector well, while an upper separated horizontal wellbore can be usedas a producer well. Multiple lower and upper horizontal wellbores can beutilized.

Obviously, many other variations and modifications of this invention aspreviously set forth may be made without departing from the spirit andscope of this invention, as those skilled in the art readily understand.Such variations and modifications are considered part of this inventionand within the purview and scope of the appended claims.

What is claimed is:
 1. A method for recovering hydrocarbonaceous fluidsfrom a formation penetrated by at least two horizontal wellscomprising:(a) injecting liquid carbon dioxide through at least onelower horizontal well into said formation at a pressure insufficient tofracture said formation; (b) thereafter injecting steam into said lowerhorizontal well, thereby causing said liquid carbon dioxide to convertto its gaseous state and expand thereby making a substantially bettersweep of the formation; (c) recovering hydrocarbonaceous fluids, gaseouscarbon dioxide, steam and water from said formation via at least oneupper horizontal well; and (d) repeating steps (a), (b) and (c).
 2. Themethod as recited in claim 1 where fluids recovered from step (c) areseparated.
 3. The method as recited in claim 1 where the API gravity ofhydrocarbonaceous fluids in said formation prior to carbon dioxideinjection is from about 10 to about 60 API degrees.
 4. A method forrecovering hydrocarbonaceous fluids from a formation penetrated by atleast two horizontal wells comprising:(a) injecting liquid carbondioxide through at least one lower horizontal well into said formationat a pressure insufficient to fracture said formation; (b) thereafterinjecting steam into said lower horizontal well, thereby causing saidliquid carbon dioxide to convert to its gaseous state and expand therebymaking a substantially better sweep of the formation; (c) repeatingsteps (a) and (b); (d) recovering hydrocarbonaceous fluids, gaseouscarbon dioxide, steam and water from said formation via at least oneupper horizontal well; and (e) separating the fluids recovered from stepd.
 5. The method as recited in claim 4 where the API gravity ofhydrocarbonaceous fluids in said formation prior to carbon dioxideinjection is from about 10 to about 60 API degrees.